Purpose: Perception, Movement, Learning, Memory, Thinking, Communication Functions:

Transcription

1 Nervous System

2 Purpose: Perception, Movement, Learning, Memory, Thinking, Communication Functions: Sensory Input: Obtaining stimulation from the environment (light, heat, pressure, vibration, chemical, electrical) Integration: Interpretation of the Signal Motor Output: Response to the Interpretation

3 Sensory input Sensor Integration Motor output Figure 48.3 Effector Peripheral nervous system (PNS) Central nervous system (CNS)

4 Basic Components Central Nervous System (CNS): Brain and the spinal cord integration and output Peripheral Nervous System (PNS): Nerves that transfer sensory and motor signals from the body to the CNS and visa versa

5 Basis of the Nervous System = Neuron (nerve cell) Structure: Cell Body: segment of the cell that contains the nucleus and organelles Dendrites: branching ends of the neuron that receive and transmit signals to the rest of the cell: many branches increases area to receive signals Axons: conduct messages to the tips of the cells - may be very long - axons branch into synaptic terminals - relay signals to other nerve cells or effector cells (muscle, secretory cells) across the synapse (space between two communicating cells)

6 Dendrites Cell body Nucleus Axon Axon hillock Signal direction Synapse Presynaptic cell Myelin sheath Postsynaptic cell Figure 48.5 Synaptic terminals

7 Axon can be coated by an insulating cover called the Myelin Sheath - formed from Schwann cells in the PNS and from Oligodendrocytes in the CNS - Myelin grows around the axon in layers like a cinnamon roll - speeds transmission of the nerve signal - made of a lipid material that insulates the axon from the electrical signals of the nerve cell - this causes the nerve signal to jump over the myelinated areas to the spaces between the myelin cluster the spaces are called the Nodes of Ranvier

8 Myelin Sheath and Multiple Sclerosis Axon` Myelin sheath Schwann cell Nodes of Ranvier Node of Ranvier Layers of myelin Schwann cell Nucleus of Schwann cell Axon Figure µm

9 Types of Neurons Sensory: communicate information both internally and externally Interneurons: Integrate sensory input and motor output Motor Neurons: Convey impulses for movement These make up circuits to convey messages throughout the body.

10 Simplest Nerve Circuit Reflex: Knee Jerk Reaction Tap on the knee puts pressure on the tendons and the muscle Pressure is sensed by a sensory receptor in the quadriceps Receptors transmit a signal to a ganglia (cluster of nerves) in the spinal cord A signal is sent to the motor neurons to contract the quadiceps and to interneurons that suppress the motor neurons of the hamstrings Forward motion of the lower leg

11 2 Sensors detect 3 Sensory neurons a sudden stretch in the quadriceps. convey the information to the spinal cord. Quadriceps muscle Cell body of sensory neuron in dorsal root ganglion 4 The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward. White matter Gray matter 5 Sensory neurons from the quadriceps also communicate with interneurons in the spinal cord. Figure The reflex is initiated by tapping the tendon connected to the quadriceps (extensor) muscle. Hamstring muscle Spinal cord (cross section) Sensory neuron Motor neuron Interneuron 6 The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps.

12 Nerve Signals Electrical signals that are established by ion differentials across the membranes of the neuron Requires a Membrane Potential a difference of charge due to different ion concentrations Membrane Potential is established by the active transport of ions, particularly Na+ and K+

13 Establishing a Membrane Potential Intracellular Ions: Principle Cation: K+, some Na+ Anions: Cl-, Neg Proteins, Amino Acids, Sulfates (SO 4 2- ), Phosphates (PO 4 3- ) Extracellular Ions: Principle Cation: Na+, some K+ Anions: some Cl-

14 Membrane potential is primarily established by the movement of sodium and potassium using the sodium/potassium pump Transmembranal Protein that pumps 3 Na+ out and 2 K+ in Results in a relative negative charge in the interior of the cell it is less positive than the outside Negative anions add to the negative character of the interior This establishes the RESTING POTENTIAL a nerve cell ready to generate a signal

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16 Transmission of a Nerve Signal -Formation of an Action Potential 1. Stimulus stimulus must be strong enough to start the process - this is known as the Threshold Potential 2. Depolarization stimulus triggers gated ion channels to open so Na+ions flow into the cell - this alters the charge difference across the membrane

17 Membrane potential (mv) The generation of an action potential Na + Na + Na + Na + Figure Rising phase of the action potential Depolarization opens the activation gates on most Na + channels, while the channels activation gates remain closed. Na + influx makes the inside of the membrane positive with respect to the outside. Na + Na + Na + Cytosol Sodium channel 2 Depolarization A stimulus opens the activation gates on some Na + channels. Na + influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. Extracellular fluid Plasma membrane Potassium channel 1 Resting state The activation gates on the Na + and channels are closed, and the membrane s resting potential is maintained Action potential 1 Resting potential 100 Time Activation gates Threshold Inactivation gate Falling phase of the action potential The inactivation gates on most Na + channels close, blocking Na + influx. The activation gates on most channels open, permitting efflux which again makes the inside of the cell negative. Na + Na + Undershoot Both gates of the Na + channels are closed, but the activation gates on some channels are still open. As these gates close on most channels, and the inactivation gates open on Na + channels, the membrane returns to its resting state.

18 3. Rising Phase of Action Potential - Na+ gates fully open inverting the membrane potential - this triggers the next section of the neuron to begin depolarization 4. Falling Phase of Action Potential - K+ gates open and K+ transfers out of the cell - this causes the membrane potential to shift back to (+) outside and (-) inside

19 Membrane potential (mv) The generation of an action potential Na + Na + Na + Na + Figure Rising phase of the action potential Depolarization opens the activation gates on most Na + channels, while the channels activation gates remain closed. Na + influx makes the inside of the membrane positive with respect to the outside. Na + Na + Na + Cytosol Sodium channel 2 Depolarization A stimulus opens the activation gates on some Na + channels. Na + influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. Extracellular fluid Plasma membrane Potassium channel 1 Resting state The activation gates on the Na + and channels are closed, and the membrane s resting potential is maintained Action potential 1 Resting potential 100 Time Activation gates Threshold Inactivation gate Falling phase of the action potential The inactivation gates on most Na + channels close, blocking Na + influx. The activation gates on most channels open, permitting efflux which again makes the inside of the cell negative. Na + Na + Undershoot Both gates of the Na + channels are closed, but the activation gates on some channels are still open. As these gates close on most channels, and the inactivation gates open on Na + channels, the membrane returns to its resting state.

20 5. Undershoot - K+ gates close and then the Resting Potential is restored by pumping Na+ out and K+ into the cell - also known as the Refractory Period because the nerve can t be restimulated during this time (about 2 milliseconds) Why Are You Hitting Yourself?

21 Membrane potential (mv) The generation of an action potential Na + Na + Na + Na + Figure Rising phase of the action potential Depolarization opens the activation gates on most Na + channels, while the channels activation gates remain closed. Na + influx makes the inside of the membrane positive with respect to the outside. Na + Na + Na + Cytosol Sodium channel 2 Depolarization A stimulus opens the activation gates on some Na + channels. Na + influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. Extracellular fluid Plasma membrane Potassium channel 1 Resting state The activation gates on the Na + and channels are closed, and the membrane s resting potential is maintained Action potential 1 Resting potential 100 Time Activation gates Threshold Inactivation gate Falling phase of the action potential The inactivation gates on most Na + channels close, blocking Na + influx. The activation gates on most channels open, permitting efflux which again makes the inside of the cell negative. Na + Na + Undershoot Both gates of the Na + channels are closed, but the activation gates on some channels are still open. As these gates close on most channels, and the inactivation gates open on Na + channels, the membrane returns to its resting state.

22 Conduction of Nerve Signals Depolarization of one region causes the section next to it to reach the threshold potential and depolarize Continues in a domino effect with the region behind the wave becoming repolarized Repolarization prevents the wave from moving backwards.

23 Axon Action potential Na + 1 An action potential is generated as Na + flows inward across the membrane at one location. Figure Action potential Na + Action potential + Na + 2 The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as flows outward. 3 The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

24 Increasing Conduction Speed Saltatory Conduction (video) Jumping of the nerve signal over the myleinated areas of the axon Schwann cell Depolarized region (node of Ranvier) Myelin sheath Figure Cell body Axon

25 Crash Course #1 Crash Course #2 Crash Course #3

26 Communication Between Nerve Cells Nerve signals must cross extracellular space at the terminal ends of the axons Space between neurons or neurons and effector cells is called the Synapse

27 Presynaptic cell Postsynaptic cell Synaptic vesicles containing neurotransmitter Presynaptic membrane 5 Na + Neurotransmitter Postsynaptic membrane Voltage-gated Ca 2+ channel Ligandgated ion channel 1 Ca Postsynaptic membrane 6 Synaptic cleft 3 Figure Ligand-gated ion channels

28 Types: 1. Electrical Synapse: Two cells bridge the synapse with Gap Junctions allowing a direct transmission from cell to cell 2. Chemical Synapse: Two cells are not connected and the signal is transmitted by the release and acceptance of chemicals called Neurotransmitters

29 Chemical Synapses Pre-synaptic cell contains synaptic vesicles filled with neurotransmitters. The action potential reaches the terminal end and causes the influx of Calcium ions Calcium ions cause the exocytosis of neurotransmitters through the merging of the synaptic vesicles with the axon membrane

30 Presynaptic cell Postsynaptic cell Synaptic vesicles containing neurotransmitter Presynaptic membrane 5 Na + Neurotransmitter Postsynaptic membrane Voltage-gated Ca 2+ channel Ligandgated ion channel 1 Ca Postsynaptic membrane 6 Synaptic cleft 3 Figure Ligand-gated ion channels

31 Neurotransmitters move across the synaptic space (cleft) and bind to gated ligand channels on the post-synaptic cell. The gated ligand channels open allowing the influx of Na+ stimulating the formation of a new action potential in the post-synaptic cell. This usually requires multiple synaptic events. Neurotransmitter is then released from the gated channel and then diffuses from the cleft, is reabsorbed or broken down by an enzyme. VIDEO Crash Course #2 Neurotransmission Animation

32 EPSP vs. IPSP EPSP Excitatory Postsynaptic Potential generated by gated ion channels that cause Na+ influx multiple EPSPs will generate a new action potential in a post-synaptic cell Example: Acetylcholeine IPSP Inhibitory Postsynaptic Potential generated by gated ion channels that cause K+ influx prevents the formation of a new potential Example: gamma-amino butyric acid (GABA) help regulate fear and anxiety

33 Neurotransmitters Acetylcholine: Between motor neurons and muscle cells stimulates muscle contraction Also acts in CNS (central nervous system) Mimicked by Nicotine

34 Biogenic Amines: from amino acids Epinephrine adrenaline increases heart and respiration rate Fight or Flight Norepinephrine similar to epinephrine increases attention and alertness Dopamine and Serotonin (from tryptophan) sleep, mood, attention and learning LSD binds to dopamine and serotonin receptors causing hallucinations

35 Caffeine: - competitively competes for adenosine binding sites in the brain - accumulation of adenosine leads to tiredness and lack of concentration - dilates blood vessels in brain and muscles increases alertness and physical performance - relaxes smooth muscle tissue - breathe easier

36 Comparative Nervous Systems Cnidarians nerve net Nerve net Figure 48.2a (a) Hydra (cnidarian)

37 Echinoderms Nerve Ring Radial nerve Nerve ring Figure 48.2b (b) Sea star (echinoderm)

38 Central Nervous Systems and Peripheral Nervous System CNS: Brain and Nerve Cord PNS: Nerves Branching off Nerve Cord

39 Eyespot Brain Nerve cord Transverse nerve Brain Ventral nerve cord Segmental ganglion Brain Ventral nerve cord Segmental ganglia (c) Planarian (flatworm) (d) Leech (annelid) (e) Insect (arthropod) Anterior nerve ring Ganglia Brain Longitudinal nerve cords Brain Ganglia Spinal cord (dorsal nerve cord) Sensory ganglion (f) Chiton (mollusc) (g) Squid (mollusc) (h) Salamander (chordate)